Background Relating to Optical Amplifiers
Single-mode rare-earth-doped optical fiber amplifiers have been widely used for over a decade to provide diffraction-limited optical amplification of optical pulses. Because single mode fiber amplifiers generate very low noise levels, do not induce modal dispersion, and are compatible with single mode fiber optic transmission lines, they have been used almost exclusively in telecommunication applications.
The amplification of high peak-power pulses in a diffraction-limited optical beam in single-mode optical fiber amplifiers is generally limited by the small fiber core size that needs to be employed to ensure single-mode operation of the fiber. In general the onset of nonlinearities such as self-phase modulation lead to severe pulse distortions once the integral of the power level present inside the fiber with the propagation length exceeds a certain limiting value. For a constant peak power P inside the fiber, the tolerable amount of self-phase modulation .PHI..sub.nl is given by ##EQU1##
where A is the area of the fundamental mode in the fiber, .lambda. is the operation wavelength, L is the fiber length and n.sub.2 =3.2.times.10.sup.-20 m.sup.2 /W is the nonlinear refractive index in silica optical fibers.
As an alternative to single-mode amplifiers, amplification in multi-mode optical fibers has been considered. However, in general, amplification experiments in multi-mode optical fibers have led to non-diffraction-limited outputs as well as unacceptable pulse broadening due to modal dispersion, since the launch conditions into the multi-mode optical fiber and mode-coupling in the multi-mode fiber have not been controlled.
Amplified spontaneous emission in a multi-mode fiber has been reduced by selectively exciting active ions close to the center of the fiber core or by confining the active ions to the center of the fiber core. U.S. Pat. No. 5,187,759, hereby incorporated herein by reference. Since the overlap of the low-order modes in a multi-mode optical fiber is highest with the active ions close to the center of the fiber core, any amplified spontaneous emission will then also be predominantly generated in low-order modes of the multi-mode fiber. As a result, the total amount of amplified spontaneous emission can be reduced in the multi-mode fiber, since no amplified spontaneous emission is generated in high-order modes.
As an alternative for obtaining high-power pulses, chirped pulse amplification with chirped fiber Bragg gratings has been employed. One of the limitations of this technique is the relative complexity of the set-up.
More recently, the amplification of pulses to peak powers higher than 10 KW has been achieved in multi-mode fiber amplifiers. See U.S. Pat. No. 5,818,630, entitled Single-Mode Amplifiers and Compressors Based on Multi-Mode Fibers, assigned to the assignee of the present invention, and hereby incorporated herein by reference. As described therein, the peak power limit inherent in single-mode optical fiber amplifiers is avoided by employing the increased area occupied by the fundamental mode within multi-mode fibers. This increased area permits an increase in the energy storage potential of the optical fiber amplifier, allowing higher pulse energies before the onset of undesirable nonlinearities and gain saturation. To accomplish this, that application describes the advantages of concentration of the gain medium in the center of the multi-mode fiber so that the fundamental mode is preferentially amplified. This gain-confinement is utilized to stabilize the fundamental mode in a fiber with a large cross section by gain guiding.
Additionally, that reference describes the writing of chirped fiber Bragg gratings onto multi-mode fibers with reduced mode-coupling to increase the power limits for linear pulse compression of high-power optical pulses. In that system, double-clad multi-mode fiber amplifiers are pumped with relatively large-area high-power semiconductor lasers. Further, the fundamental mode in the multi-mode fibers is excited by employing efficient mode-filters. By further using multi-mode fibers with low mode-coupling, the propagation of the fundamental mode in multi-mode amplifiers over lengths of several meters can be ensured, allowing the amplification of high-power optical pulses in doped multi-mode fiber amplifiers with core diameters of several tens of microns, while still providing a diffraction limited output beam. That system additionally employed cladding pumping by broad area diode array lasers to conveniently excite multi-mode fiber amplifiers.
Background Relating to Modelocked Lasers
Both actively modelocked lasers and passively modelocked lasers are well known in the laser art. For example, compact modelocked lasers have been formed as ultrashort pulse sources using single-mode rare-earth-doped fibers. One particularly useful fiber pulse source is based on Kerr-type passive modelocking. Such pulse sources have been assembled using widely available standard fiber components to provide pulses at the bandwidth limit of rare-earth fiber lasers with GigaHertz repetition rates.
Semiconductor saturable absorbers have recently found applications in the field of passively modelocked, ultrashort pulse lasers. These devices are attractive since they are compact, inexpensive, and can be tailored to a wide range of laser wavelengths and pulsewidths. Quantum well and bulk semiconductor saturable absorbers have also been used to modelock color center lasers.
A saturable absorber has an intensity-dependent loss l. The single pass loss of a signal of intensity I through a saturable absorber of thickness d may be expressed as EQU l=1-exp(-.alpha.d)
in which .alpha. is the intensity dependent absorption coefficient given by: EQU .alpha.(I)=.alpha..sub.0 /(1+I/I.sub.SAT)
Here .alpha..sub.0 is the small signal absorption coefficient, which depends upon the material in question. I.sub.SAT is the saturation intensity, which is inversely proportional to the lifetime (.tau..sub.A) of the absorbing species within the saturable absorber. Thus, saturable absorbers exhibit less loss at higher intensity.
Because the loss of a saturable absorber is intensity dependent, the pulse width of the laser pulses is shortened as they pass through the saturable absorber. How rapidly the pulse width of the laser pulses is shortened is proportional to .vertline.dq.sub.0 /dI.vertline., in which q.sub.0 is the nonlinear loss: EQU q.sub.0 =l(I)-l(I=0)
l(I=0) is a constant (=1-exp(-.sub.0 d)) and is known as the insertion loss. As defined herein, the nonlinear loss q.sub.0 of a saturable absorber decreases (becomes more negative) with increasing intensity I. .vertline.dq.sub.0 /dI.vertline. stays essentially constant until I approaches I.sub.SAT becoming essentially zero in the bleaching regime, i.e., when I&gt;&gt;I.sub.SAT.
For a saturable absorber to function satisfactorily as a modelocking element, it should have a lifetime (i.e., the lifetime of the upper state of the absorbing species), insertion loss l(I=0), and nonlinear loss q.sub.0 appropriate to the laser. Ideally, the insertion loss should be low to enhance the laser's efficiency, whereas the lifetime and the nonlinear loss q.sub.0 should permit self-starting and stable cw modelocking. The saturable absorber's characteristics, as well as laser cavity parameters such as output coupling fraction, residual loss, and lifetime of the gain medium, all play a role in the evolution of a laser from startup to modelocking.
As with single-mode fiber amplifiers, the peak-power of pulses from mode-locked single-mode lasers has been limited by the small fiber core size that has been employed to ensure single-mode operation of the fiber. In addition, in mode-locked single-mode fiber lasers, the roundtrip nonlinear phase delay also needs to be limited to around .pi. to prevent the generation of pulses with a very large temporally extended background, generally referred to as a pedestal. For a standard mode-locked single-mode erbium fiber laser operating at 1.55 .mu.m with a core diameter of 10 .mu.m and a round-trip cavity length of 2 m, corresponding to a pulse repetition rate of 50 MHz, the maximum oscillating peak power is thus about 1 KW.
The long-term operation of mode-locked single-mode fiber lasers is conveniently ensured by employing an environmentally stable cavity as described in U.S. Pat. No. 5,689,519, entitled Environmentally Stable Passively Modelocked Fiber Laser Pulse Source, assigned to the assignee of the present invention, and hereby incorporated herein by reference. The laser described in this reference minimizes environmentally induced fluctuations in the polarization state at the output of the single-mode fiber. In the described embodiments, this is accomplished by including a pair of Faraday rotators at opposite ends of the laser cavity to compensate for linear phase drifts between the polarization eigenmodes of the fiber.
Recently the reliability of high-power single-mode fiber lasers passively mode-locked by saturable absorbers has been greatly improved by implementing nonlinear power limiters by insertion of appropriate semiconductor two-photon absorbers into the cavity, which minimizes the peak power of the damaging Q-switched pulses often observed in the start-up of mode-locking and in the presence of misalignments of the cavity. See U.S. patent application Ser. No. 09/149,369, filed on Sep. 8, 1998, is now an U.S. Pat. No. 6,158,031 filed on Dec. 5, 2000 entitled Resonant Fabry-Perot Semiconductor Saturable Absorbers and Two-Photon Absorption Power Limiters, assigned to the assignee of the present invention, and hereby incorporated herein by reference.
To increase the pulse energy available from mode-locked single-mode fiber lasers the oscillation of chirped pulses inside the laser cavity has been employed. M. Hofer et al., Opt. Lett., vol. 17, page 807-809. As a consequence the pulses are temporally extended, giving rise to a significant peak power reduction inside the fiber laser. However, the pulses can be temporally compressed down to approximately the bandwidth limit outside the laser cavity. Due to the resulting high peak power, bulk-optic dispersive delay lines have to be used for pulse compression. For neodymium fiber lasers, pulse widths of the order of 100 fs can be obtained.
The pulse energy from mode-locked single-mode fiber lasers has also been increased by employing chirped fiber gratings. The chirped fiber gratings have a large amount of negative dispersion, broadening the pulses inside the cavity dispersively, which therefore reduces their peak power and also leads to the oscillation of high-energy pulses inside the single-mode fiber lasers.
See U.S. Pat. No. 5,450,427, entitled Technique for the Generation of Optical Pulses in Mode-Locked Lasers by Dispersive Control of the Oscillation Pulse Width, and U.S. Pat. No. 5,627,848, entitled Apparatus for Producing Femtosecond and Picosecond Pulses from Fiber Lasers Cladding Pumped with Broad Area Diode Laser Arrays, both of which are assigned to the assignee of the present invention and hereby incorporated herein by reference. In these systems, the generated pulses are bandwidth-limited, though the typical oscillating pulse widths are of the order of a few ps.
However, though the dispersive broadening of the pulse width oscillating inside a single-mode fiber laser cavity does increase the oscillating pulse energy compared to a `standard` soliton fiber laser, it does not increase the oscillating peak power. The maximum peak power generated with these systems directly from the fiber laser is still limited to around I KW.
Another highly integratable method for increasing the peak power of mode-locked lasers is based on using chirped periodically poled LiNb03 (chirped PPLN). Chirped PPLN permits simultaneous pulse compression and frequency doubling of an optically chirped pulse. See U.S. patent application No. 08/845,410, filed on Apr. 25, 1997, is now an U.S. Pat. No. 5,867,304 filed on Feb. 2, 1999 entitled Use of Aperiodic Quasi-Phase-Matched Gratings in Ultrashort Pulse Sources, assigned to the assignee of the present application, and hereby incorporated herein by reference. However, for chirped PPLN to produce pulse compression from around 3 ps to 300 fs and frequency doubling with high conversion efficiencies, generally peak powers of the order of several KW are required. Such high peak powers are typically outside the range of mode-locked single-mode erbium fiber lasers.
Broad area diode laser arrays have been used for pumping of mode-locked single-mode fiber lasers, where very compact cavity designs were possible. The pump light was injected through a V-groove from the side of double-clad fiber, a technique typically referred to as side-pumping. However, such oscillator designs have also suffered from peak power limitations due to the single-mode structure of the oscillator fiber.
It has also been suggested that a near diffraction-limited output beam can be obtained from a multi-mode fiber laser when keeping the fiber length shorter than 15 mm and selectively providing a maximum amount of feedback for the fundamental mode of the optical fiber. "Efficient laser operation with nearly diffraction-limited output from a diode-pumped heavily Nd-doped multimode fiber", Optics Letters, Vol. 21, pp. 266-268 (1996) hereby incorporated herein by reference. In this technique, however, severe mode-coupling has been a problem, as the employed multi-mode fibers typically support thousands of modes. Also, only an air-gap between the endface of the multi-mode fiber and a laser mirror has been suggested for mode-selection. Hence, only very poor modal discrimination has been obtained, resulting in poor beam quality.
While the operation of optical amplifiers, especially in the presence of large seed signals, is not very sensitive to the presence of spurious reflections, the stability of mode-locked lasers critically depends on the minimization of spurious reflections. Any stray reflections produce sub-cavities inside an oscillator and result in injection signals for the cw operation of a laser cavity and thus prevent the onset of mode-locking. For solid-state FabryPerot cavities a suppression of intra-cavity reflections to a level &lt;&lt;1% (in intensity) is generally believed to be required to enable the onset of mode-locking.
The intra-cavity reflections that are of concern in standard mode-locked lasers can be thought of as being conceptually equivalent to mode-coupling in multi-mode fibers.
Any mode-coupling in multi-mode fibers clearly also produces a sub-cavity with a cw injection signal proportional to the amount of mode-coupling. However, the suppression of mode-coupling to a level of &lt;&lt;I% at any multi-mode fiber discontinuities is very difficult to achieve. Due to optical aberrations, even well corrected optics typically allow the excitation of the fundamental mode in multi-mode fibers only with maximum efficiency of about 95%. Therefore to date, it has been considered that mode-locking of a multi-mode fiber is impossible and no stable operation of a mode-locked multi-mode fiber laser has yet been demonstrated.